Method of producing compound oxide superconducting thin film

ABSTRACT

A method for producing a compound oxide superconducting thin film, comprising forming an oxide thin film on the surface of a substrate of a first metal element having a redox charge by oxidizing the metal, using the oxide thin film thus formed as an electrode for oxidation reaction of a second metal element contained in an electrolyte solution or molten salt to incorporate the second metal element in the oxide thin film, using the compound oxide thin film thus formed as an electrode to obtain a cyclic voltammogram, and electrochemically processing the compound oxide thin film at an electrolytic potential that is determined based on the cyclic voltammogram.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of producing a compound oxidesuperconducting thin film, and more particularly to a method ofproducing a superconducting thin film of a compound oxide containingcopper, bismuth, vanadium or other such element having a redox charge.

2. Prior Art Statement

Following the discovery by J. G. Bednorz and K. A. Muller of theoriginal oxide superconductor consisting primarily of copper, a numberof other oxide superconductors have been developed which use theincorporation and combination of various metal oxides to increase thecritical temperature at which superconductivity is exhibited. However,while numerous attempts have been made to develop new superconductors,the lack of any theoretical guidelines for increasing thesuperconducting critical temperature means that researchers engaged insuch development have to rely on the experience they accrue throughtheir work.

The oxide superconductors discovered up to now include among theirconstituent metal elements at least one element having a redox charge.Moreover, it is known that the element or elements are simultaneouslypresent in different charge states at different portions of one and thesame superconductor and that where these different charge states areregulated by oxygen, the oxygen atoms are usually not present in aninteger ratio to the metal atoms. It is therefore thought that thispresence of oxygen atoms at a non-integer ratio, or the variation invalence number of metal element atoms, has an effect on thecharacteristics of the superconductor and its critical temperature.

In the case of a superconductor consisting primarily of a copper oxide,methods that have been used to increase the critical temperature atwhich superconductivity is exhibited include raising the average valencenumber of the copper by using a flow of gas having an oxygen contentthat is higher than the atmospheric oxygen content or using a flow ofoxygen gas itself, and increasing the ratio of copper (III) in oxygengas under high pressure.

Superconductors have also been produced in which a substance having aredox charge such as oxides of bismuth (V), rare earth metals, alkalineearth metals and the like have been added to the copper in certainamounts to increase the valence number of the copper.

Thus, such superconductors have been produced by starting with acombination of two or more oxides. However, when the superconductors areformed as thin films for electronic device applications, there have beena number of problems, such as that it is difficult to achieve continuityof superconducting portions, so superconductivity is not readilyexhibited.

With respect to most of the high-temperature superconductors that havebeen previously developed, carrier injection is considered to be animportant factor that determines the critical temperature T_(c) at whichsuperconductivity is exhibited. A carrier injection method disclosed inU.S. Pat. No. 5,286,711 by the present inventors comprised producing anoxide superconductor by an electrochemical oxidation-reduction (redox)method in which the electrolytic potential for the electrochemicalprocessing is determined on the basis of a cyclic voltammogram.

The object of this invention is to provide a method for producing acompound oxide superconducting thin film having a high criticaltemperature.

SUMMARY OF THE INVENTION

For achieving this object, the present invention provides a method forproducing a compound oxide superconducting thin film, comprising formingan oxide thin film on the surface of a substrate of a first metalelement having a redox charge by oxidizing the metal, using the oxidethin film thus formed as an electrode for oxidation reaction of a secondmetal element contained in an electrolyte solution or molten salt toincorporate the second metal element in the oxide thin film, using thecompound oxide thin film thus formed as an electrode to obtain a cyclicvoltammogram, and electrochemically processing the compound oxide thinfilm at an electrolytic potential that is determined based on the cyclicvoltammogram.

This method according to the present invention uses the metal oxide thinfilm first formed as an electrode for the oxidation reaction of anothermetal element contained in an electrolyte solution or molten salt tothereby incorporate the other metal element into the oxide thin film,and an electrolytic processing potential that is determined based on thecyclic voltammogram, whereby superconductivity-imparting oxygen carriersand defects thus introduced into the metal oxide thin film by the othermetal element result in a superconducting thin film having an increasedcritical temperature. The metal element may be supported on a substrate.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and followingdetailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the arrangement of an electrolysis apparatus used toproduce a compound oxide thin film according to an embodiment of thepresent invention;

FIG. 2 is a cyclic voltammogram of a lanthanum-copper-oxygen (La-Cu-O)oxide thin film; and

FIG. 3 shows an x-ray diffraction curve of a La-Cu-O oxide thin filmobtained by the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the method for producing a compound oxide superconducting thin filmaccording to this invention, a substrate of a first metal element havinga redox charge is electrochemically oxidized to form a thin film of anoxide of the metal on the surface of the substrate, and the oxide thinfilm thus formed is then used as an electrode for an oxidation reactionin an electrolyte solution or molten salt containing another metalelement to be used to form a desired compound by being incorporated intothe oxide thin film. The compound oxide thin film thus formed is used asan electrode to derive a cyclic voltammogram, and the compound oxidethin film is then electrolyzed at an electrolytic potential determinedon the basis of the cyclic voltammagram to thereby achievesuperconductivity.

Copper, silver and bismuth are metal elements having a redox charge. Thesubstrate may be constituted by a thin layer of one of these metalelements, or a conventional electrochemical deposition method may beused to constitute the substrate as a thin layer of one of these metalelements on a base of ceramic, carbon, high polymer, glass,semiconductor or other such material able to withstand a temperature ofaround 250° C. in the case of an aqueous solution, or around 500° C. inthe case of a molten salt, and a certain degree of pressure.

Electrochemical oxidation is then used to form a thin film of an oxideof the metal on the substrate surface. This oxidation may be done usinga known electrolysis method or by a method involving the coating of asubstance that is readily oxidized.

Following the formation of the oxide thin film, the metal substrate isused as a positive electrode and platinum as a negative electrode toeffect an oxidation reaction in an electrolyte solution or molten saltcontaining one or more metal ions to form the oxide thin film into acompound oxide thin film.

The electrolyte used for the above oxidation reaction is a saturatedsolution containing the metal used to form the compound oxide at a highconcentration of 1M or above. The metal may be a hydroxide or salt ofone or more elements that will enable the bulk critical temperatureT_(c) to be increased, such as for example lanthanum, yttrium, barium,strontium, thallium, mercury, lead, silver, and bismuth.

While the oxidation potential varies depending on the metal oxide, as aguide it is a positive potential that is higher than the standardoxidation potential used to produce an oxide of the metal concerned (forexample, 1 to 10 volts). In the case of a slow reaction, theelectrolytic cell is placed in a sealed container and the reaction takesplace under elevated temperature and pressure conditions, at around roomtemperature to 200° C. when the electrolyte is an aqueous solution andat around 50° to 500° C. when the electrolyte is a molten salt. Thisresults in the intercalation or inversion of the compound metal elementor elements in the metal oxide thin film.

If lanthanum ions are being introduced into a copper substrate, thereaction will be as follows. ##STR1##

The compound oxide thin film thus obtained is used as an electrode toobtain a cyclic voltammogram in the same electrolyte.

The criteria used when the electrolytic potential for impartingsuperconductivity to the oxide thin film is determined from the cyclicvoltammogram are (1) that there is a flow of oxidation current orreduction current, (2) that at the same potential as in the above (1)there is a large difference between the oxidation current value and thereduction current value, and (3) that oxygen gas is generated at thesame potential.

The compound oxide thin film is then subjected to electrolysis at thepotential determined based on the cyclic voltammogram, to therebyproduce the superconductivity. The electrolysis period is usually up toone hour. The valence number of the oxide element or elements having aredox charge is controlled by this electrolysis to produce an oxidesuperconducting thin film. The present inventors found that compared toprior to the electrolysis there is an increase or decrease in the amountof oxygen in a thin film superconductor that has a higher criticaltemperature T_(c) following the electrolysis. Therefore, by using thethus obtained compound oxide superconducting thin film having a redoxcharge as an electrode to generate oxygen or hydrogen under prescribedvoltage conditions to thereby increase or decrease the amount of oxygenin the compound oxide superconducting thin film used as the electrode, acompound oxide superconducting thin film can be obtained.

X-ray diffraction images of the structure of the oxide superconductingthin film exhibiting a high critical temperature T_(c) after thusadjusting the valence number of elements or the amount of oxygen in theoxide thin film showed that in the case of La-Cu-O oxide, for example,there was an increase in the split width on the X-ray spectrogram. Inthe case of La-Cu-O, this means that the critical temperature T_(c) canbe increased by applying electrolysis that produces an increase of thesplit width on the X-ray diffraction image.

The operation and effect of the invention will now be described withreference to an oxide superconducting thin film in which the metalelement having a redox charge is copper.

When the surface of a copper substrate used as the positive electrode issubjected to electrolytic oxidation in an aqueous solution, hydroxidemolten salt or nitrogen fused salt, in accordance with the followingreaction a thin film of oxide is formed on the surface of the coppersubstrate.

    Cu(S)+H.sub.2 O→CuO+H.sub.2 ↑+2e(neutral)     (2)

    Cu(S)+2OH.sup.- →Cu(OH).sub.2 (alkaline)            (3)

Moreover, copper in an alkaline solution or alkaline molten salt willnormally form a hydroxide, but if it is electrochemically oxidized at apotential higher than 0.5 volt (using a calomel electrode as thereference electrode), the copper will become trivalent in accordancewith the following redox reaction.

    Cu(OH).sub.2 =CuO.sub.2.sup.- +H.sub.2 O+e.sup.-           (4)

The present inventors found that when a copper substrate is immersed ina potassium hydroxide solution and the oxidation reaction is induced onthe copper surface under the above conditions, using an iron (II) ionredox reaction to detect the amount of copper (III) from the aboveaqueous solution showed the average valence of copper in the oxide is ina high oxidation state. A copper substrate with the surface thusoxidized remained stable in air for at least one week.

When an oxide coated copper substrate is used as the positive electrodeand platinum as the negative electrode to perform an oxidation reactionin an aqueous solution or molten salt containing ions of barium andlanthanum as the metal to be added to form the compound, as shown by thefollowing formula, the reaction is promoted, introducing the barium andlanthanum ions into the oxide thin film to form a compound oxide thinfilm. This compound oxide thin film is then used as an electrode toobtain a cyclic voltammogram, and the cyclic voltammogram is then usedas a basis for determining the electrolytic potential to be used tothereby produce a compound oxide superconducting thin film with a highcritical temperature. ##STR2##

In an electrolyte solution or molten salt, copper usually dissolvesmainly as copper (I) or copper (II); because of the powerful oxidizingeffect, copper (III) is not stable in such a solution or molten salt.However, in the case of the present invention, even if the copper oxideused in the superconducting thin film does not dissolve in the solutionor molten salt, it is exposed on the surface thereof and oxidized in anondissolved state as copper (III) in the solution or molten salt towhich a voltage is applied.

As has been described in the foregoing, the present invention enables acompound oxide superconducting thin film having a high criticaltemperature to be obtained by using an oxide thin film of a metalelement having a redox charge as an electrode for oxidation reaction ofanother metal element contained in an electrolyte solution of moltensalt not causing dissolution of the oxide thin film to obtain a compoundoxide thin film, using the compound oxide thin film as an electrode toobtain a cyclic voltammogram and electrochemically processing thecompound oxide thin film at an electrolytic potential determined on thebasis of the cyclic voltammogram, and makes it possible to continuouslyproduce the compound oxide thin film, obtain the cyclic voltammogram andelectrochemically process the compound oxide thin film to exhibitsuperconductivity in a single electrolytic cell.

Furthermore, the present invention can be utilized as a simple methodfor causing a metal element having a redox charge and contained in anoxide thin film to exhibit superconductivity by controlling the numberof charges of the metal element. Moreover, according to the presentinvention, an oxide superconducting thin film which has been difficultto produce can be obtained with ease.

EXAMPLE 1

A magnesium oxide substrate was coated with a copper (II) compound thatwas then dried in air and oxidized by heating and used as an electrode,immersed in an electrolyte solution consisting of a saturated solutionof barium hydroxide containing a suspension of about twice the amount oflanthanum nitrate •6H₂ O. This was heated to about 50° C. as oxidationwas carried out by applying a voltage of 4 volts across the electrodes,producing a thin film of copper-barium-lanthanum-oxide (Cu-Ba-La-O) onthe surface of the magnesium oxide substrate.

FIG. 1 shows the electrolysis apparatus used in the method according tothis invention. Reference numeral 1 denotes an electrolytic cellcontaining a 1.0M solution of KOH as the electrolyte. Electrolysis isperformed by passing a voltage of 0.8 to 1.0 volts between a workingelectrode 2 constituted by the Cu-Ba-La oxide thin film, and a platinumelectrode 3. The applied potential was controlled by a potentiostat 5using a saturated calomel electrode 4 as a reference electrode.

The thin film of Cu-Ba-La-O thus processed was then used as an electrodeto obtain the cyclic voltammogram in the above suspension solution. Thecyclic voltammogram thus obtained is shown in FIG. 2. The oxidation ofcopper (II) to copper (III) in the electrolysis process, followed by theproduction of oxygen, were clearly observed.

This cyclic voltammogram shows that there is a large difference betweenthe oxidation current and the reduction current at 0.7 to 0.8 volts, so0.8 volts (vs. the saturated calomel electrode) was determined as anoxidation potential, and the Cu-Ba-La-O thin film was thereforeelectrolyzed for about 0.5 hours at this potential, producing an oxidesuperconducting thin film having a thickness of about 500 nm. This thinfilm was then removed from the electrolytic cell 1 and washed, and thecritical temperature T_(c) was measured and found to have increased byabout 30 degrees Kelvin. FIG. 3 is an X-ray diffraction spectrogram ofthe thin film, showing that an increase in the split widths (204) and(024), and (133) and (313) was recognized under the higher criticaltemperature T_(c) compared to before the electrolysis.

EXAMPLE 2

A molten mixture of barium hydroxide •8H₂ O and yttrium nitrate •6H₂ Owas prepared. While the melting temperature varies depending on theproportions of each of the salts, this mixture melts at a lowtemperature of about 70° to 100° C. Copper was then deposited on amagnesium oxide substrate and oxidized, and the copper was then used asan electrode for electrolytic processing in a molten salt mixtureconsisting of 20 percent barium hydroxide and 80 percent yttriumnitrate. The electrolysis conditions were substantially the same asthose used in the first example, and the electrolysis voltage was 8volts. The oxide thus obtained was washed and examined, confirming thatthe thin film was a compound oxide superconducting thin film containingbarium and yttrium.

Japanese Patent Application No. 5-98692 filed Apr. 1, 1993 is herebyincorporated by reference.

What is claimed is:
 1. A method for producing a compound oxidesuperconducting thin film, comprising:forming a thin oxide film on thesurface of a substrate comprising a first metal selected from the groupconsisting of copper, silver and bismuth having a redox charge byoxidizing the first metal; using the thin oxide film thus formed as anelectrode for an oxidation reaction of a second metal element containedin an electrolyte solution or molten salt electrolyte to incorporate thesecond metal element in the thin oxide film; using the compound thinoxide film thus formed as an electrode to obtain a cyclic voltammogram;and electrochemically processing the compound thin oxide film at anelectrolytic potential that is determined from the cyclic voltammogramof the thin oxide film which is the potential at which there is a largedifference between the oxidation current value and the reduction currentvalue of the thin oxide film, so that the film exhibitssuperconductivity.
 2. The method according to claim 1, wherein thesubstrate comprises a layer of the first metal supported on a base of adifferent material.
 3. The method according to claim 2, wherein thesubstrate base is formed of a member selected from the group consistingof ceramic, carbon, high polymer, glass and semiconductor.
 4. The methodaccording to claim 1, wherein the substrate comprises a layer of coppersupported on a base of a different material.
 5. The method according toclaim 4, wherein the substrate base is formed of a member selected fromthe group consisting of ceramic, carbon, high polymer, glass andsemiconductor.
 6. The method according to claim 1, wherein the oxidationreaction takes place at room temperature to 200° C. when the electrolyteis an aqueous solution and at 50° to 500° C. when the electrolyte is amolten salt.